feat: add StaticFileFS (#2749)

* RouterGroup.StaticFileFS added
add StaticFileFS ad README

* fix Static content mistake

* update README `tab`
improve StaticFile and StaticFileFS code, use staticFileHandler

routergroup.go

@@ -44,6 +44,7 @@ type IRoutes interface {
 	HEAD(string, ...HandlerFunc) IRoutes
 
 	StaticFile(string, string) IRoutes
+	StaticFileFS(string, string, http.FileSystem) IRoutes
 	Static(string, string) IRoutes
 	StaticFS(string, http.FileSystem) IRoutes
 }
@@ -153,12 +154,24 @@ func (group *RouterGroup) Any(relativePath string, handlers ...HandlerFunc) IRou
 // StaticFile registers a single route in order to serve a single file of the local filesystem.
 // router.StaticFile("favicon.ico", "./resources/favicon.ico")
 func (group *RouterGroup) StaticFile(relativePath, filepath string) IRoutes {
+	return group.staticFileHandler(relativePath, func(c *Context) {
+		c.File(filepath)
+	})
+}
+
+// StaticFileFS works just like `StaticFile` but a custom `http.FileSystem` can be used instead..
+// router.StaticFileFS("favicon.ico", "./resources/favicon.ico", Dir{".", false})
+// Gin by default user: gin.Dir()
+func (group *RouterGroup) StaticFileFS(relativePath, filepath string, fs http.FileSystem) IRoutes {
+	return group.staticFileHandler(relativePath, func(c *Context) {
+		c.FileFromFS(filepath, fs)
+	})
+}
+
+func (group *RouterGroup) staticFileHandler(relativePath string, handler HandlerFunc) IRoutes {
 	if strings.Contains(relativePath, ":") || strings.Contains(relativePath, "*") {
 		panic("URL parameters can not be used when serving a static file")
 	}
-	handler := func(c *Context) {
-		c.File(filepath)
-	}
 	group.GET(relativePath, handler)
 	group.HEAD(relativePath, handler)
 	return group.returnObj()